On a serious note, when riding with someone of lesser ability than yourself (for everyone here, that means anyone aside from Pros), a mirror is quite useful to help you maintain the max pace that your riding partner can manage. It's very easy to keep constant tabs on their gap to your rear wheel without losing focus on the road ahead.

I use a Take-A-Look on my (sun)glasses. Feels weird to ride without a mirror these days after more than decade with one.